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Klein-Gordon equation in quantum field theory is known to suffer from the possibility of negative probability. So, the question is, despite this, Klein-Gordon describes spin-zero field. So, how can negative probability and scalar field co-exist?

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More on Klein-Gordon equation and negative probabilities in quantum mechanics: physics.stackexchange.com/q/39224/2451 – Qmechanic♦ Oct 14 '12 at 18:20

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In QFT, we reinterpret the probability density as the probability charge density. In other words, negative probabilities correspond to antiparticles.

In fact, the Dirac equation which describes spin-1/2 also has this property, and it led to the prediction of the positron as the antiparticle of the electron.

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but Wikipedia and physics.stackexchange.com/q/39224/2451 say that negative probability density do not make sense, and this is the reason for developing Dirac equation... So.. what is that and this? – War Oct 14 '12 at 22:52
Yes, negative probabilities do not make sense. What physicists discovered was that they were not calculating the probability density, but ~ probability density * charge. So if charge is opposite, they get a negative answer, but the actual probability is positive definite. – hwlin Oct 15 '12 at 0:25

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